1. Field of the Invention
The present invention relates to an opto-electric hybrid board including an optical waveguide portion, an electric circuit portion, and an optical element mounted on this electric circuit portion, and to a manufacturing method thereof.
2. Description of the Related Art
An opto-electric hybrid board is constructed, for example, as shown in FIG. 7, by bonding an electric circuit portion 6 and an optical waveguide portion 7 to each other with an adhesive 5, and then mounting a light-emitting element 3 and a light-receiving element 4 on the above-mentioned electric circuit portion 6. The above-mentioned optical waveguide portion 7 includes an optical waveguide 70 in which an under cladding layer 71, a core 72, and an over cladding layer 73 are disposed in the order named as seen from the above-mentioned electric circuit portion 6 side. The opposite end portions of this optical waveguide 70 are formed as inclined surfaces inclined at 45 degrees to an optical axis, and core 72 portions of the respective inclined surfaces are formed as light reflecting surfaces 72a. The above-mentioned electric circuit portion 6 is constructed by forming an electric circuit 61 on one surface of a substrate 60. Part of the electric circuit 61 serves as mounting pads 61a for mounting the light-emitting element 3 and the light-receiving element 4 described above thereon. The above-mentioned substrate 60 is formed with light-passing through holes 62 and 63 for propagation of light L between the end portions of the core 72 and the light-emitting element 3 and light-receiving element 4. In FIG. 7, the reference character 3a designates a bump (an electrode) for the above-mentioned light-emitting element 3, and the reference character 4a designates a bump for the above-mentioned light-receiving element 4.
The propagation of the light L in the above-mentioned opto-electric hybrid board is as follows. First, the light L is emitted downwardly from the light-emitting element 3. The light L passes through the under cladding layer 71 in a first end portion (a left-hand end portion as seen in FIG. 7) of the optical waveguide 70, and then enters a first end portion of the core 72. Subsequently, the light L is reflected from the light reflecting surface 72a provided in the first end portion of the core 72, and travels through the interior of the core 72 in an axial direction. Then, the light L travels through the interior of the core 72, and is propagated to a second end portion (a right-hand end portion as seen in FIG. 7) of the core 72. Subsequently, the light L is reflected upwardly from the light reflecting surface 72a provided in the above-mentioned second end portion, passes through and exits from the under cladding layer 71, and is received by the light-receiving element 4. Thus, the accurate positioning of the light-emitting element 3 and the light-receiving element 4 relative to the opposite end portions of the core 72 of the optical waveguide 70 is important in achieving high light propagation efficiency.
To this end, there has been proposed a method of manufacturing an opto-electric hybrid board in which alignment marks 24 serving as a reference for the positioning of the light-emitting element 3 and the light-receiving element 4 are formed in an optical waveguide portion 2 so that the light-emitting element 3 and the light-receiving element 4 described above are positioned relative to the opposite end portions of a core 22 of an optical waveguide 20, as shown in FIG. 8A (see, for example, Japanese Patent Application Laid-Open No. 2004-302345). This manufacturing method includes: forming an under cladding layer 21 as the optical waveguide portion 2 with reference to FIG. 8A; thereafter forming a photosensitive resin layer having a region in which the core 22 is to be formed and a region in which the alignment marks 24 are to be formed on a surface (the lower surface as seen in the figure) of the under cladding layer 21; and then forming the core 22 and the alignment marks 24 from the photosensitive resin layer by a photolithographic process. As shown in FIG. 8B, an example of each of the alignment marks 24 is formed in the shape of a disk including in its central position a through hole 24a having the shape of a cross as seen in plan view. The cross-shaped portion serves as an identifying mark. Then, a liquid material for the formation of an over cladding layer 23 is applied to the uncovered surfaces of the under cladding layer 21, the core 22 and the alignment marks 24 described above, and is then hardened by exposure to light or the like, whereby the over cladding layer 23 is formed. At this time, the inside of each of the above-mentioned cross-shaped through holes 24a is also filled with the liquid material for the formation of the above-mentioned over cladding layer 23, and becomes part of the over cladding layer 23. In this manner, the alignment marks 24 are formed in predetermined positions relative to the opposite end portions of the core 22 together with the optical waveguide 20. On the other hand, a substrate 80 is prepared which is formed with light-passing through holes 82 and 83 and through holes 84 for the recognition of the above-mentioned alignment marks 24. Then, the above-mentioned substrate 80 is affixed to the upper surface of the under cladding layer 21 of the above-mentioned optical waveguide portion 2 with the adhesive 5, and an electric circuit 81 (including mounting pads 81a) is formed on the upper surface of the substrate 80 by a photolithographic process using the above-mentioned alignment marks 24 as a reference. Thus, an electric circuit portion 8 is produced on the above-mentioned optical waveguide portion 2, with the adhesive 5 lying therebetween. Thereafter, the light-emitting element 3 and the light-receiving element 4 are mounted on the mounting pads 81a. In this method, the above-mentioned mounting pads 81a are formed with reference to the alignment marks 24 formed in predetermined positions relative to the opposite end portions of the above-mentioned core 22. Therefore, the above-mentioned mounting pads 81a are positioned relative to the opposite end portions of the core 22.
However, there is a danger that the light-emitting element 3 and the light-receiving element 4 deviate from the above-mentioned mounting pads 81a during the mounting of the light-emitting element 3 and the light-receiving element 4 on the above-mentioned mounting pads 81a. To avoid the danger, Hodono, the inventor of the present application has proposed a method of manufacturing an opto-electric hybrid board in which, during the mounting of the light-emitting element 3 and the light-receiving element 4, as shown in FIG. 9, the above-mentioned optical waveguide portion 2 and the electric circuit portion 8 bonded together are set on a stage S of a mounting machine, the above-mentioned alignment marks 24 are recognized by means of an alignment recognition device C provided in the mounting machine, and the mounting is achieved using the alignment marks 24 as a reference, and has already applied for a patent (Japanese Patent Application No. 2008-114329) (U.S. patent application Ser. No. 12/428,669). Thus, the positioning of the light-emitting element 3 and the light-receiving element 4 described above is accomplished more properly.
In this manner, the above-mentioned alignment marks 24 are generally made of the material for the formation of the core 22 by the photolithographic process at the same time that the core 22 of the optical waveguide 20 is formed from the viewpoint of the positioning relative to the end portions of the core 22. The alignment marks 24 made of the material for the formation of the core 22 are embedded in the over cladding layer 23 at the surface of the under cladding layer 21, and the cross-shaped portion in the central position of each of the above-mentioned alignment marks 24 serves as part of the over cladding layer 23. As a result of the nature of the optical waveguide 20, the alignment marks 24 made of the material for the formation of the core 22 are translucent, and the under cladding layer 21 and the over cladding layer (including the cross-shaped portions) 23 are generally also translucent. Additionally, there is a difference in refractive index between the alignment marks 24 (approximately 1.588 as the refractive index of the core 22) and the under cladding layer 21 and over cladding layer 23 (having a refractive index of approximately 1.502 to 1.542), but the difference in refractive index is small (approximately 0.05 to 0.09).
For this reason, both the cross-shaped portions of the above-mentioned alignment marks 24 and their surrounding portions are recognized as being bright, and the difference in brightness therebetween is small. As a matter of fact, it is difficult to see the cross-shaped portions of the above-mentioned alignment marks 24 through the under cladding layer 21 by using a pattern matching scheme employed for the alignment recognition device C of the mounting machine (a scheme such that an image is converted into coordinates and numerals by the numerical conversion of the contrast between black and white; a gray scale pattern recognition scheme), as shown in FIG. 9. Additionally, the surface (the uncovered surface exposed to the through holes 84 for the recognition of the alignment marks 24; the upper surface as seen in the figure) of the under cladding layer 21 is uneven. Light for illumination or the like is reflected diffusely because of the unevenness, and it tends to be difficult to obtain an image having a constant contrast. For this reason, the recognition takes much time, and the mounting step requires a prolonged period of time. Also, there is apprehension that false recognition results. In this regard, there is room for improvement.